Black films for metal surfaces

ABSTRACT

A black, smutlike film, resistant to rub off, and uniform in appearance and thickness, is deposited on a metal surface by contacting the surface with an aqueous acidic solution containing bismuth, hydrogen, nitrate, and sulfate ions. The blackened surface is rendered corrosion resistant by further treatment with a phosphating solution to deposit a phosphate conversion coating thereon.

United States Patent OTHER REFERENCES Hopkins, The Scientific AmericanCyclopedia of Formulas, I925, Scientific American Pub. Co., p. 439

Fishlock, Metal Coloring, I962, Robert Draper L.T.D., p. 242

Vozdvizhenskii, Chem. Abstract, Vol. 29, 1935, p. 3918 PrimaryExaminer-Ralph S. Kendall Assistant Examiner-Caleb WestonAttorneys-Cedric H. Kuhn, Robert E. Dunn, Bernhard R.

Swick, Joseph D. Michaels and Charles G. Lamb ABSTRACT: A black,smutlike film, resistant to rub off, and

uniform in appearance and thickness, is deposited on a metal surface bycontacting the surface with an aqueous acidic solution containingbismuth, hydrogen, nitrate, and sulfate ions. The blackened surface isrendered corrosion resistant by further treatment with a phosphatingsolution to deposit a phosphate conversion coating thereon.

BLACK FILMS FOR METAL SURFACES The present invention concerns coatingsfor metal surfaces. More particularly, the present invention concerns animproved method for producing corrosion-resistant blackened ferrousmetal surfaces. Even more particularly, the present invention relates toan improved bath for depositing a strongly adherent, black film onferrous metal surfaces.

The use of blackened, corrosionresistant ferrous metal surfaces forimparting decorative appearances to a variety of assemblies and partshas found wide application in the automotive industry. The prior art isreplete with compositions and techniques adapted to provide theseblackened, corrosion-resistant surfaces. Initially, the art revealsmethods wherein a phosphated metal surface is subsequently coated witheither a black paint, stain, ink, or dye which is introduced onto thesurface either in an aqueous medium or as a sealant in a rustpreventiveoil. These methods of coloring enjoy only moderate success because ofthe nonuniformity, susceptibility to fading, and nonpermanenceoccasioned with their use.

Today, one of the common methods for providing blackened, corrosionresistant surfaces comprises a two-step procedure of l blackening thesurface and then (2) applying a protective coating to the blackenedsurface. The blackening step is usually carried out by immersing themetallic surface or substrate in an aqueous acidic solution havingnitrate and either bismuth or antimony ions contained therein. Theblack, smutlike film deposited thereby is then fixed to the surface bytreatment with a zinc phosphate solution which simultaneously rendersthe substrate corrosion resistant. Although the films in and ofthemselves do not afford adequate corrosion resistance, bismuth filmshave been found to provide nearly twice the corrosion resistance ofantimony. And while it is the subsequent phosphating of the surface thatimparts the bulk of the corrosion resistance to the surfaces, it wouldbe desirable to take advantage of the increased corrosion resistanceimparted by the bismuth film.

Although both bismuth films and antimony films are subject to removal bymanual rubbing bismuth films, disadvantageously, are susceptible to ruboff even under running tap water exerting a pressure of less than 1p.s.i.g. However, because bismuth films afford nearly twice thecorrosion resistance of antimony filmsa solution to the problem of ruboff would provide an important advance in the art.

Accordingly, it is an object of the present invention to provide animproved bismuth black film for ferrous metal surfaces which isresistant to rub 05 upon subjection to a running water tap and whichpresents a unifonn thickness and appearance. A further object of thepresent invention is to provide an improved method for producingbismuth-blackened corrosion-resistant metal surfaces. It will becomeapparent to those skilled in the art that these and other objects areachieved by the present invention on reference to the following detaileddescription.

in accordance with the present invention, a ferrous metal surface, suchas iron, steel alloys and the like, is rendered corrosion resistant byfirst contacting the surface with an aqueous acidic solution containingbismuth, nitrate, hydrogen and sulfate ions to deposit a black, smutlikefilm thereon. Although it is feasible to utilize the present inventionwith other metals which can receive a phosphate coating, the presentinvention is particularly applicable to ferrous metals. Thereafter, theblackened surface is contacted with a phosphate solution to deposit aphosphate conversion protective coating thereon. Optionally, thephosphate coating may then be passivated by treatment with chromic acid,or chromic-phosphoric acid. The coating is then usually sealed with arust-preventive oil, wax, water-emulsifiable oil or the like.

ln one embodiment, the present invention contemplates a novel bath fordepositing black films on metal surfaces. It has now unexpectedly beendiscovered that by incorporating concentrations of sulfate ion into anaqueous acidic bismuth and nitrate ion containing solution, a black filmwhich has improved resistance to rub off is imparted to metal surfacescontacted therewith. This discovery has obviated the drawbacks to theuse of bismuth films that were heretofore encountered.

The sulfate ion utilized in the present bath can be introduced intosolution in any convenient manner, such as by a sulfate salt or an acid.It is also within the purview of the invention to add to the acidicsolution suitable sulfate ion producing reagents, such as the complex ofdimethyl thiourea and nitric acid. However, because of economicconsideration, it is preferred to employ the sulfate salts or acid. Asnoted, any sulfate salt is suitable for use in preparing thefilm-forming solutions of the invention. Examples of such salts includebismuth sulfate, calcium sulfate, sodium sulfate, potassium sulfate, andthe like. However, because of its availability and low cost, sulfuricacid is the most practical sulfate ion source to use. Additionally,sulfuric acid is an excellent source of free hydrogen ions in thepresent solutions.

The effective concentration of the sulfate ion in the acidic solutionhas been found to be variable over a wide range. Concentrations rangingfrom about O.l5 to 8.0 percent of sulfate based on the weight of thesolution are effective. Preferably from about 1.0 to 5.0 percent byweight of the solution is comprised of the sulfate ion.

The bismuth ion can be present in the bath either as a salt or as theanhydrous powder metal. Any bismuth salt can be employed, such as,bismuth chloride, bismuth sulfate, bismuth nitrate, bismuth oxide, andthe like. Preferably, the bismuth ion is introduced into the acidicsolution as an anhydrous metal powder. Concentrations ranging from 0.0lto l .0 weight percent of the ion in an acid solution having from 0.0lto 0.2 weight percent of hydrogen ion are preferred. Concentrations ofthe hydrogen ion greater than 0.2 weight percent promote pickling andare to be avoided.

In formulating the baths within the above-defined percentages, it isnecessary to maintain certain weight ratios of the ions. These ratiosmay be summarized as follows:

1. There must be a minimum sulfate ion to bismuth ion weight ratio offrom 7.7:] to l5.4:l, and usually at least about I02]. If the weightratio falls below the understood minimum, then the adhesion of thebismuth film to the metal substrate is quite poor. The maximum weightratio of sulfate ion to bismuth ion is limited only by economicconsiderations. Generally, however, from about a 202i to 50:l weightratio can be used, preferably from about 301i to 40: l.

2. The weight ratio of hydrogen ion to bismuth ion is critical, forotherwise the bismuth ion does not go into solution or is incompletelysolubilized. It has been found that the hydrogen ion to bismuth ionweight ratio must be at least I113 and cannot exceed about l:l.3 sinceat higher ratios the metal substrate is subject to pickling.

3. The nitrate radical which is introduced into the solution as nitricacid is present in a weight ratio of l.5:l to 2.5:l times that of thebismuth concentration, preferably about 2.0:1. Hence, the nitrateradical comprises from about 0.15 to 2.5 percent by weight of thesolution and preferably from 0.2 to 2 percent by weight of the solution.The balance of the solution comprises water.

Optionally, a stannous ion, comprising about 0.00l to [.0 percent byweight of the solution, may be incorporated into the bath. The stannousion accelerates and enhances deposition of the bismuth film on the metalsurface. Preferably, the stannous ion can be provided by a stannoussalt, such as stannous chloride which ionizes in acidic solutions.

When a metal surface is contacted. preferably by immersion, with thisnovel bath for a period of about 0.25 to ID minutes and at a temperatureof about 20 to 35 C., a black, smutlike film, resistant to rub off undera running water tap exerting pressures of greater than 35 p.s.i.g. andof uniform thickness and appearance, is deposited on the metal surface.

After the smutlike film is deposited on the metal surface a protectivecoating is thereafter applied. Any one of a variety of well-knowncoatings can be used. It is preferred, however, to apply a phosphatecoating to the blackened surface. The

phosphate coatings vary widely in composition and they generallycomprise aqueous acidic solutions containing phosphate, zinc, nitrate,and ferrous ions. It is, usually, advantageous to have at least one ionselected from the group of calcium, nitrate, chlorate, lithium, cobalt,and nickel ions present as an auxiliary ion. Any such conventionalsolution having phosphate, zinc, nitrate and ferrous ions and in whichthere is, preferably, incorporated nickel ions is contemplated by thepresent invention. These solutions are commercially available and aresatisfactory for use in practicing the present invention.

The phosphate coatings are generally applied by an immersion technique.The conventional procedure generally associated with their applicationcomprises contacting the blackened surface with about a 3 to 4 percentby weight aqueous acidic zinc phosphate solution at about 60 to 95 C.for about 5 to 30 minutes and thereafter rinsing and drying the surfaceby conventional techniques. The phosphate coating that is depositedfixes the black film to the surface and simultaneously renders thesurface corrosion resistant.

Optionally, the zinc phosphate coating can be passivated. This entailsthe application of a coating of either a chromic acid or achromic-phosphoric acid (modified chromic acid) solution to thephosphated surface. Such chromating solutions are widely known andcommercially available, usually, in concentrated solutions. Diluteaqueous solutions of these concentrated chromating solutions rangingfrom 6 to 16 ounces of concentrate per 100 gallons (gal.) of solution;preferably 8 to 12 ounces of concentrate per I gal. of solution areconventionally utilized to passivate the conversion coating.

Usually, an immersion technique is employed to passivate the phosphatecoating although spray techniques can be utilized for light metalobjects. Generally, the blackened, phosphated surface is immersed in anaqueous bath, such as described above, for a period of from 15 to 60seconds, desirably about 30 seconds at elevated temperatures of about 70to 90 C., preferably at about 75C.

If the surface is passivated it may either be rinsed and dried or mayhave a sealant directly applied to it. If the surface is not passivated,then a sealant is directly applied. In any event, the final step in theprocess comprises the sealing of the blackened, phosphated surface.

As in known in the art any suitable organic oil, wax wateremulsifiableoil or the like will suffice as a sealant for the phosphate coating.

Again, in applying the sealant, an immersion technique is generallyemployed. The surface is immersed in an aqueous solution usually havingfrom about to 30 percent by volume of the sealant contained therein fora period of about 1 to minutes and at elevated temperatures of about 70to 90 C. The high temperature promotes drainage and dry-off of anyexcess sealant on the surface.

The following examples, which are not to be construed as undulylimitative of the present invention, set forth specific embodiments ofthe present invention which exhibit the heretofore defined qualities andproperties. Unless otherwise indicated all percentages are by weight.

EXAMPLE I A 2 inch X4 inch 1010 carbon steel panel was immersed at about24 C. in a black trlm bath which has the following composition:

Wt. '5 of Solution Bit! (introduced as anhydrous metal powder) 0.65 SO](introduced or ,SO.) 7.98 N0,

(introduced I: FIND L30 H 0 (introduced as "NO, and H,S0,) 0.20 ",089.87

After about 0.25 minutes the surface was completely and uniformlyblackened. The panel was then subjected to a running water tap exertinga pressure of at least 35 p.s.i.g. The black film remained deposited onthe metal substrate with no evidence of rub off.

EXAMPLE II A 2 inch X4 inch I010 carbon steel panel was immersed at 24C. in a black film bath having the following composition:

Wt. of Solution After 3 minutes a black film was deposited in the panel.The film did not rub off under a water pressure of 35 p.s.i.g.

EXAMPLE Ill The procedure of example I was followed except that the bathhad the following composition:

\Vt. it of Solution The black film was deposited in 4 minutes.Comparable results were obtained when the blackened surface wassubjected to a running water tap exerting a pressure of 35 p.s.i.g.

EXAMPLE lV Again, the procedure of example I was followed except thatthe bath had the following composition:

Wt. i of Solution (introduced a anhydrous metal powder) 0.0325 S0,

(introduced at 8.80,) 1.0000 N0,

(introduced an VINO.) 0.0650 "0 (introduced as SO, and HNO.) 0.0250 098.8775

Testing for rub off under the same conditions as in example I gavecomparable results.

EXAMPLE V The procedure of example I was followed except that the blackfilm bath had the following composition:

Wt. 11 of Solution Biol (introduced as anhydrous metal powder) 0.0l625SO.= (introduced as H,SO 0.50000 NO, (introduced as HNO,) 0.03250(introduced as H,SO and HNO,) 0.01250 ",0 99.43875 Testing for rub offunder the same conditions as in example I gave comparable results.

EXAMPLE V! EXAMPLE VII To illustrate the improved adherence of thepresent films, a 2 inch 4 inch lOlO carbon steel panel was immersed at24 C. in a black film bath having the following composition:

Wt. I: of Solution (introduced as anhydrous metal powder 0.l30

(introduced as HNO,) 9.600

(introduced as HNO,) 0.095

After 3.5 minutes a black film was deposited on the panel.

When the blackened panel was subjected to a running water tap pressureof l p.s.i.g., the black film washed off. Hence without the presence ofthe sulfate ion in the bath, the deposited black films are readilysusceptible to rub off under running water tap pressures well belowthose of the films deposited from sulfate ion containing baths.

What is claimed is:

l. A bath for depositing a black film on a ferrous metal surfaceconsisting essentially of:

a. from about 0.01 to 1.0 percent by weight of bismuth ions;

b. from about 0.015 to 2.5 percent by weight of nitrate ions;

0. from about 0.0] to 0.20 percent weight of hydrogen ions;

d. from about 0.25 to 8.0 percent by weight of sulfate ions;

e. from about 99.715 to 89.30 percent by weight of water; and whereinthe minimum sulfate ion to bismuth ion weight ratio is from 7.7:] tol5.4:l.

2. The bath of claim 1 wherein:

a. said bismuth ion is present in a weight ratio to said hydrogen ionoffrom 13:1 to 3.25:];

b. said sulfate ion is present in a weight ratio to said bismuth ion ofat least 10: l and c. said nitrate ion is present in a weight ratio tosaid bismuth ion offrom l.5:l to 2.5: l.

3. The bath of claim 1 and further including from about 0.001 to 1.0percent by weight of a stannous ion.

4. A method for producing a blackened, corrosion-resistant ferrous metalsurface comprising:

a. contacting said surface at about 20 to 30 C. for about 0.25 to 10minutes with an aqueous acidic bath comprismg:

I. from about 0.01 to 1.0 percent by weight of a bismuth ion;

2. from about 0.015 to 2.50 percent by weight of a nitrate ion;

3. from about 0.01 to 0.20 percent by weight of a hydrogen ion;

4. from about 0.25 to 8.00 percent by weight of a sulfate ion;

5. from about 99.715 to 89.30 percent by weight of water; and whereinthe minimum sulfate ion to bismuth ion weight ratio is from 7.7:l tol5.4:l, thereby depositing a black film on said surface;

b. thereafter contacting said blackened surface with a zinc phosphatingsolution, thereby depositing a phosphate conversion coating thereon; and

c. sealing said coated surface by contacting said surface with asealant.

5. The method of claim 4 wherein prior to said sealant being applied,said surface is passivated by contacting said surface with an aqueousacidic solution selected from the group consisting of chromic acid andchromic-phosphoric acid.

6. A blackened, corrosion-resistant ferrous metal surface prepared inaccordance with the method of claim 4.

7. A blackened, corrosion-resistant ferrous metal surface prepared inaccordance with the method of claim 5.

2. from about 0.015 to 2.50 percent by weight of a nitrate ion;
 2. Thebath of claim 1 wherein: a. said bismuth ion is present in a weightratio to said hydrogen ion of from 13:1 to 3.25:1; b. said sulfate ionis present in a weight ratio to said bismuth ion of at least 10:1; andc. said nitrate ion is present in a weight ratio to said bismuth ion offrom 1.5:1 to 2.5:1.
 3. The bath of claim 1 and further including fromabout 0.001 to 1.0 percent by weigHt of a stannous ion.
 3. from about0.01 to 0.20 percent by weight of a hydrogen ion;
 4. from about 0.25 to8.00 percent by weight of a sulfate ion;
 4. A method for producing ablackened, corrosion-resistant ferrous metal surface comprising: a.contacting said surface at about 20* to 30* C. for about 0.25 to 10minutes with an aqueous acidic bath comprising:
 5. from about 99.715 to89.30 percent by weight of water; and wherein the minimum sulfate ion tobismuth ion weight ratio is from 7.7:1 to 15.4:1, thereby depositing ablack film on said surface; b. thereafter contacting said blackenedsurface with a zinc phosphating solution, thereby depositing a phosphateconversion coating thereon; and c. sealing said coated surface bycontacting said surface with a sealant.
 5. The method of claim 4 whereinprior to said sealant being applied, said surface is passivated bycontacting said surface with an aqueous acidic solution selected fromthe group consisting of chromic acid and chromic-phosphoric acid.
 6. Ablackened, corrosion-resistant ferrous metal surface prepared inaccordance with the method of claim
 4. 7. A blackened,corrosion-resistant ferrous metal surface prepared in accordance withthe method of claim 5.